It is known to provide a so-called Smart Remote Actuator (SRA), which is an electro-mechanical device that is configured to actuate or manipulate an attached device, such as for example an engine feature like a position of a variable geometry turbocharger (VGT) or a position of an exhaust gas recirculation (EGR) valve. In one construction, the SRA includes a brushless direct current (BLDC) motor, whose rotor is coupled to a gear-train, which in turn is coupled to an output shaft. The output shaft of the SRA is configured to be coupled by way of intermediate linkage such as a second gear train or a lever-connecting rod-lever combination or the like to transfer the rotation of the output shaft to a desired movement of the manipulated device (e.g., rotation of an EGR valve throughout its operating range). The SRA may include integrated rotary position detection hardware/software that is configured to provide an indicated position of the actuator as feedback.
It is known to use an SRA in a VGT application. In this application, the actuator operates the VGT over a range of travel that avoids physical contact with the mechanical travel limits, except when those travel limits are deliberately contacted for the purpose of learning their locations. For this application, not contacting the travel limits was acceptable. However, for other applications, such as an EGR valve application, the precision needed required that the mechanical travel limits sometimes be contacted, perhaps for indefinite periods of time. A problem with contacting the mechanical travel limits, especially for extended periods of time, is that the phase currents provided to the motor would overheat the motor and perhaps damage it. This is due in part to the characteristics of the motor. In operation, when the rotor is rotating, a back electromagnetic force (BEMF) is produced which tends to reduce the amount of current that flows through the coils for a given applied voltage. However, in a stall condition, as would exist when contacting a travel limit, only the resistance of the coils themselves (generally low) impedes current flow, which can result in an excessive current situation. It is this excessive current that produces undesirable amounts of heat—potentially damaging the motor and/or associated packaging/components.
There is therefore a need for a method for controlling a holding force against travel limit positions that minimizes or eliminates one or more of the problems noted above.